Threaded joints are widely used for connecting OCTG and other steel pipes used in oil industry facilities. Typically, standard threaded joints as specified in American Petroleum Institute (API) standards have been used to connect steel pipes that are used to prospect for and to produce oil and gas. However, the drilling/production environment is becoming increasingly harsh in recent years because crude oil wells and natural gas wells are becoming deeper and the number of horizontal wells and directional wells are increasing relative to the number of vertical wells. Because development of wells in hostile environments such as oceans and polar regions is increasing, various characteristics such as resistance to compression, resistance to bending, sealability against external pressure (external pressure resistance) are required for a threaded joint. Therefore, an increasing number of special high-performance threaded joints, which are called premium joints, are being used, and the demand for improving the characteristics of such joints has been increasing.
Typically, a premium joint is a coupling-type joint in which a pair of externally threaded members (hereinafter referred to as pins) formed at ends of pipes and an internally threaded member (hereinafter referred to as a box) are coupled. Each of the pins includes a tapered thread, a seal portion (to be specific, a metal-to-metal seal portion), and a shoulder portion (to be specific, a torque shoulder portion). The box connects the pins to each other. The tapered thread is important for strongly fixing the pipe joint. The seal portion serves to secure sealability because the box and the pins come into metal-to-metal contact with each other in this portion. The shoulder portion forms a shoulder face that serves as an abutment when the joint is being fastened.
FIG. 5 schematically illustrates a premium joint for OCTG, showing a longitudinal section of a threaded joint for circular pipe. The threaded joint includes pins 3 and a box 1 corresponding to the pins 3. Each of the pins 3 includes an externally threaded portion 7 formed on an outer face thereof and a nose 8 (pin nose 8). The nose 8 is an unthreaded portion disposed at an end of the pin 3 and adjacent to the externally threaded portion 7. The nose 8 includes a seal portion 11 on an outer peripheral face thereof and a shoulder portion 12 on an end face thereof. The box 1, which faces the pin 3, has an inner face including an internally threaded portion 5, a seal portion 13, and a shoulder portion 14, which are respectively capable of mating with or coming into contact with the externally threaded portion 7, the seal portion 11, and the shoulder portion 12 of the pin 3. The numeral 15 denotes a load flank face.
Galling resistance and sealability are both required for a threaded joint for OCTG. Galling resistance is a property with which galling does not occur when the threaded joint is fastened and when it is fastened and loosened repeatedly. Sealability is a property with which leakage of fluid from the inside and inflow of fluid from the outside do not occur. However, galling resistance and sealability are generally incompatible with each other. For example, the probability of occurrence of galling can be reduced by decreasing the seal interference amount. In this case, however, metal-to-metal contact of the seal portion becomes weak and the probability of occurrence of leakage and inflow of fluid increases. For achieving sealability, using a radial seal type joint in which a pin or a box has a small seal taper angle is advantageous. However, when the seal taper angle is reduced, the sliding distance when fastening the joint increases and galling tendency increases.
In order to prevent galling, a coating is formed on a sliding face to reduce friction of metal-to-metal contact between the pin and the box. Also for a premium joint for OCTG, a coating is formed on the inner face of the box to prevent occurrence of galling when fastening the joint. For example, Patent Literature 1 describes forming a coating on a metal seal portion of one of a pin and a box of a threaded joint to achieve galling resistance.